The present invention relates to a liquid crystal display device, and more particularly to a so-called active matrix type liquid crystal display device.
Recently, a liquid crystal display device has been popularly used as a display equipment of image information and character information for an information equipment represented by a personal computer, a portable information terminal, a portable telephone, or a visual equipment such as digital camera or a VTR equipment with a built-in camera or the like.
Recently, along with the spreading of large capacity media brought about by the advent of DVDs and the rapid progress of large-capacity magnetic drives and the starting of BS digital broadcasting, the fusion of personal computers and video digital media has been in progress and the demand for an image display device of high image quality which can cope with these applications has been increasing.
A liquid crystal display adopting an in-plane switching (IPS) mode has been admitted as a display method which can satisfy the demand for high image quality and various improvements have been made to obtain the further enhancement of the image quality.
Here, the IPS mode liquid crystal display is a display which is constituted such that on each liquid-crystal-side pixel region of one substrate out of a pair of substrates which are arranged to face each other in an opposed manner by way of liquid crystal therebetween, a pixel electrode and a counter electrode which generates an electric field between the pixel electrode and the counter electrode are mounted, and the light transmittance of liquid crystal is controlled by a component of the electric field which is parallel to the substrates.
On the other hand, along with the spreading of portable telephones and portable information terminals, the demand for intermediate-sized or miniaturized liquid crystal display devices of extremely small power consumption has been also increasing.
With respect to the IPS mode liquid crystal display device, as disclosed in Japanese Patent Laid-Open No. 36058/1995, for example, a method which switches on or off liquid crystal based on a lateral electric field which is generated between metal electrodes constituting different layers byway of an insulation film is most popularly adopted. This structure, however, has a drawback that it is difficult to increase the pixel numerical aperture and hence, the light utilization efficiency is low compared to a usual TN-type display device.
However, to compensate for this drawback, it is necessary to increase the brightness of a backlight and hence, it is difficult for such a structure to achieve the low power consumption required by a notebook type personal computer or a portable terminal as a whole LCD module (the structure being referred to as “first conventional technique” hereinafter).
Further, although it is necessary to increase a distance between respective electrodes to increase the numerical aperture in the pixel constitution of the above-mentioned method, a driving voltage is elevated when the distance between electrodes is widened and hence, the power consumption of the driver LSI is increased. Accordingly, it has been difficult for the conventional technique to achieve the low power consumption of the IPS mode LCD.
To solve such a problem, Japanese Patent Laid-Open No. 316383/1999, for example, discloses a method in which the numerical aperture of the pixel is enhanced by driving liquid crystal based on a fringe electric field which is generated between a planar transparent electrode and a comb-like electrode made of a transparent electrode which is formed as a layer different from the former transparent electrode and above the former transparent layer (hereinafter referred to as “second conventional technique”).
Further, as a method for reducing a driving voltage, Japanese Patent Laid-Open No. 148596/1994, for example, discloses a method in which the driving voltage is reduced by providing two transistors which connect liquid crystal driving electrodes to pixels and these transistors are subjected to differential driving (hereinafter referred to as “third conventional technique”).
In this third conventional technique, y pieces of gate signal lines and x+1 pieces of drain signal lines are provided to the pixels arranged in a matrix array of y rows and x columns. Accordingly, the drain signal lines are commonly used by a group of pixels which belong to two neighboring columns.
First of all, the drawback of the first conventional technique lies, as mentioned previously, in that the increase of the numerical aperture is difficult because of the use of metal electrodes which form different layers by way of the insulation film as the liquid crystal driving electrodes and hence, the low power consumption cannot be achieved.
With respect to the drawback of the second conventional technique, although the second prior art can definitely enhance the numerical aperture, residual images are considerably generated compared to the conventional method and hence, it is difficult to achieve high quality images. Further, since it is necessary to form transparent electrodes which constitute respective layers during steps, the steps become complicated and hence, the reduction of cost is difficult.
With respect to the drawback of the third conventional technique, although the lowering of driving voltage can be achieved at the liquid crystal driving part, it is necessary to perform the conversion of image data in the former stage of the driver LSI such that the differential voltage is supplied between two signal lines and this makes a circuit for conversion complicated and pushes up a manufacturing cost. Further, a dynamic range of voltage in the inside of the conversion circuit is increased to the contrary compared to the usual driving and hence, the power consumption of the conversion circuit is increased whereby it is difficult to achieve the reduction of power consumption of the module as a whole.
As mentioned above, it has been difficult for the respective conventional techniques or the combination of these conventional techniques to apply the IPS mode liquid crystal to the equipment which is required to satisfy the low power consumption.
The present invention has been made to solve these drawbacks of the conventional techniques and it is an object of the present invention to provide a liquid crystal display device having a wide viewing angle which is suitably applicable to a notebook type personal computer or a portable terminal.